1. Prepare the dough. In a bowl, mix together flour, water and salt until smooth. Divide the dough into two parts.
2. Roll out each dough ball into a very thin rectangle (about 1/10 inch thick). Brush each of the dough rectangles with melted butter. Stack their tops together and brush with butter again.
3. Roll up the dough rectangle into log, keeping it fairly tight as you go. Cover with plastic and put in refrigerator for 1 hour.
4. Meanwhile, prepare the filling. Cut chicken into very small cubes (it's easier to achieve when chicken is not completely defrosted). Place it in a bowl, add finely grated onion, cilantro and cumin. Season with salt and pepper.
5. Cut the dough into 8 pieces. Put each dough piece cut side up and gently roll out into a circle about 5 inches in diameter, trying not to ruin the layers.
6. Place the filling in the center of each circle.
7. Now we need to form triangles. Dip your finger tips in flour. Bring two sides together, pinch tightly to seal and then pinch the third side to form a pyramid. Make sure all the edges are sealed well.
8. Preheat oven to 380 degrees F (190 degrees C).
9. Place samosas sealed side down on a baking sheet, previously covered with parchment paper.
10. Brush samosas with egg and sprinkle with sesame seeds.
11. Bake in the preheated oven for about 30 minutes, or until golden brown. Serve hot.